1. Technical Field
The present invention relates to colour measurement methods and to colour measurement devices.
2. Background Art
Colour measurement devices of the type under discussion can be embodied, irrespective of the underlying measurement technology, as autonomous devices or as peripheral measurement devices for use in connection with a controlling computer which evaluates measurement data. Autonomous colour measurement devices contain all the operating and display members necessary for measurement operations and also their own power supply and are in many cases also equipped with an interface for communicating with a computer, wherein both measurement data and control data can be exchanged with the computer. Colour measurement devices which are configured as peripheral measurement devices do not generally have their own operating and display members and are controlled by the superordinate computer like any other peripheral computer device. For communicating with a computer, more modern colour measurement devices are often for example fitted with a so-called USB (universal serial bus) interface, via which in many cases it is simultaneously also possible to supply power (from the attached computer).
Metallic paints and paints containing effect pigments are being used more and more nowadays, not only in the automobile industry. Such paints show a significant angular dependence. Paints containing aluminium flakes, for example, show a significant brightness flop. Paints containing interference effect pigments also show differences in colour when the observation or illumination direction is changed, for this purpose. Multi-angle measurement devices have become established for measuring such paints. Measuring brilliance is a related topic, in which the measurement result is likewise angle-sensitive.
Measurement devices which can detect such properties have to be embodied to illuminate the measurement object in one or more different, exactly defined illumination directions (nominal illumination directions) and to pick up the light reflected by the measurement object from at least one exactly defined observation direction (nominal observation direction). The observation direction and the illumination direction can be swapped in accordance with the Helmholtz reciprocity theorem. Colour measurement devices of this type are for example described in great detail in the documents EP 2 703 789 A1 and EP 2 728 342 A1.
In the publication “Device Profiling: Managing Global Colour Consistency” by Wilhelm H. Kettler, DFO Conference on Quality Assurance and Testing Methods 2008, different causes are presented which can lead to measurement errors when using such colour measurement devices. These include in particular the so-called systematic errors which are due to certain deficiencies of the device, such as for example faulty calibration. In a lecture entitled “Colour Management” given by Wilhelm H. Kettler for FARBE & LACK Seminars Module 2: Deeper Insights into Colorimetry, 25-27 Jun. 2014, Society for Research into Pigments and Paints (FPL), Stuttgart, particular reference is also made to so-called angular errors which can result from the geometrical conditions of the illumination and observation beam paths and from the apertures of the illumination and observation beam paths. Angular errors, i.e. deviations between actual illumination and observation directions of the measurement device and corresponding nominal illumination and observation directions as predetermined by the measurement geometry selected, have a particularly significant effect, precisely when performing measurements on samples containing effect paints.
The present invention deals primarily with avoiding and/or compensating for or correcting measurement errors caused by such angular errors.
Documents EP 2 703 789 A1 and EP 2 728 342 A1 describe methods and measures on how distortions in the measurement values caused by angular errors in the measurement device can be corrected. The measures described in these documents, however, require more apparatus and/or more complex measurement devices and are also relatively elaborate in purely procedural terms.
In the publication “Making Sense of Measurement Geometries for Multi-Angle Spectrophotometers” by Eric Kirchner and Werner Cramer in Color Research & Application 37.3 (2012), pages 186-198, a formalism is described regarding how paired combinations of illumination directions and observation directions are unambiguously assigned a direction in which an effect pigment flake has to be orientated in a paint in order to reflect the illumination specularly in the observation direction. This direction is referred to in the literature as the flake normal angle (cf. FIG. 3). Assigning this direction to a combination of an illumination direction and an observation direction is called “transforming into the flake angle space” in the following. The orientation of the effect pigment flake in the flake angle space, i.e. the flake normal angle, can be the same for multiple combinations of an illumination direction and an observation direction.
It is the intention of the present invention to improve a colour measurement method and a corresponding colour measurement device of the respective generic type to the effect that distortions in the measurement values caused by angular errors in the different measurement channels (illumination and observation directions) can be corrected in a relatively simple way and without adding to the complexity of the colour measurement device, such that the nominal illumination and observation directions predetermined by the respective measurement geometry are exactly maintained and distortions in the measurement values are thus avoided. Another aim is to improve the degree of match between measurement values from different colour measurement devices of the same design.